CHEM 1053: Experiments that led to nuclear theory

Dalton's Atomic Theory was developed in an attempt to explain the Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions. There are four postulates to this theory. First, each element is composed of tiny indestructible particles called atoms. Second, all atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements. Third, atoms combine in simple whole number ratios to produce compounds. And fourth, atoms of one element cannot be changed into atoms of another element. In a chemical reaction, we're only changing what atoms are bonded to what atoms, not changing atoms from one element to another. Remember that theories are refined with time. We've since revised Dalton's Atomic Theory to take into account isotopes, for example, which are atoms of the same element that have a different number of neutrons. But Dalton's Atomic Theory was a reasonable explanation of why we observe what we do with our three laws. Additional experiments have refined our understanding so that we've discovered building blocks of atoms. J.J. Thomson's cathode ray experiments led to the discovery of the electron. In the experiment, a high voltage is applied across a partially evacuated tube. As a result, streams of negatively charged particles are ejected from the cathode. These particles can be deflected with an electric or magnetic field. The particles were determined to be a fundamental component of the atom because different materials ejected the same charged particles which were much smaller than the atom. In studying the atom, early scientists applied what they knew about the properties of electric charge. For example, opposite electric charges attract and like electrical charges repel. And a positive and negative electrical charge of equal magnitude will cancel each other out so that the system is overall neutral. J.J. Thomson also found the charge-to-mass ratio of the electron by deflecting electron beams. A moving negative charge will be deflected towards the positive terminal of an externally applied electrical field. And a moving charge through a perpendicular externally applied magnetic field will also experience a force called the Lorentz force, which is perpendicular to both the externally applied magnetic field and the direction of the charge's movement. Based on the externally applied fields and the deflection, the charge-to-mass ratio of the moving charge, the electron, can be found. Robert Millikan's oil drop experiment was used to calculate the elementary charge of the electron. In this experiment, charged oil droplets are suspended using an externally applied electric field. The mass of the oil droplet can be calculated and used to determine the charge on the oil droplet. That measured charge has to be a multiple of the elementary charge of the electron. For example, an oil droplet might have 10 extra electrons on it. Then another oil droplet is used, and it might have 50 electrons on it. If you continue this process with lots of oil droplets, you can calculate the elementary charge of the electron. With the discovery of the electron, this negatively charged component of the atom, scientists knew there also had to be positive charge in the atom if it is overall electrically neutral. So J.J. Thomson proposed the plum pudding model of the atom where the electrons, negatively charged, are embedded in a sphere of positive charge, much like raisins are embedded in plum pudding. Ernest Rutherford attempted to validate this model. Rutherford devised a gold foil experiment where a very thin sheet of gold is bombarded with alpha particles. Now, with regards to radiation, alpha particles are huge. An alpha particle has a mass more than 7000 times the mass of an electron. So bombarding electrons with alpha particles is like sending a cannonball at a ping-pong ball. Rutherford expected all of the Alpha particles to just pass right through or maybe be deflected ever so slightly. And that's what he found. Well, mostly. But there were a very small percentage of alpha particles that were deflected at higher angles or even reflected back toward the source of alpha particles. Clearly, it couldn't have been the electrons that caused this. You're not going to shoot a cannonball at a ping-pong ball and have the cannonball bounce back. So a new model of the atom was needed. Namely, the nuclear model. With the nuclear model of the atom, most of the mass of the atom and all of its positive charge is located in a very dense core called the nucleus. So, most of the volume of the atom is just empty space with the very small electrons dispersed as a negative cloud around the nucleus. Electrically neutral atoms have the same number of protons, which are positively charged, in the nucleus as they have electrons. Subsequent work by Rutherford showed that there are also neutral particles in the nucleus called neutrons, which helped account for additional mass in the atom without adding additional positive charge. So our protons and neutrons are roughly the same size, and they account for the vast majority of the mass of the atom. And electrons are much smaller.